The present invention relates to a diesel engine, and in particular, relates to a combustion chamber structure of the diesel engine which can restrain the soot emission.
In the direct-injection type of diesel engine, in which fuel is injected from the fuel injector which is arranged approximately at the center of the combustion chamber at around the top dead center of compression of the piston, the cavity is formed on the top of the piston and the combustion chamber is defined by the top of the piston including the cavity, the cylinder, and the face of the cylinder head which faces the top of the piston, which is disclosed in Japanese Patent Laid-Open Publication No. 2003-254066, for example.
The above-described cavity is formed in a circular shape with its center at the fuel injector and is concaved away from the cylinder head. Its wall shape on the cross section including the center axis of the cylinder consists of, as shown in FIG. 12, for example, an inward protruding portion 2 which is located at the periphery of the opening of the top of the piston for a cavity 1, a center portion 3 which projects toward a fuel injector 5 and is located at the bottom of the cavity 1, and a peripheral portion 4 which connects the inward protruding portion 2 and the center portion 3 and is made with arcs having their centers on the inner side of the cavity.
Herein, the fuel injected from the fuel injector 5 at around the top dead center of compression of the piston directs to the proximity of the border between the inward protruding portion 2 and the peripheral portion 4 of the cavity 1. After colliding with the wall surface of the cavity 1, the fuel spreads along the wall surface of the peripheral portion 4 and toward the center portion 3. Thus, the fuel mixes with the air in the combustion chamber and starts its combustion.
If the mixing of the fuel spray from the fuel injector and the air in the cavity was so insufficient that the air-fuel ratio of the mixture did not become properly uniform (homogeneous), the soot would be generated at an over-rich area inside the combustion chamber, so that the soot would be exhausted to the outside without sufficiently oxidizing (purifying).
It is preferable that the fuel spray be mixed with the air so well in the cavity that the whole part of mixture can become properly uniform (homogeneous) in the lean state of air-fuel ratio in order to restrain the generation of the soot or promote the oxidization of the generated soot. Thus, as shown in FIG. 12, it may be necessary that the flowing direction of the fuel spray A which is injected from the fuel injector 5 and directs to the proximity of the border between the inward protruding portion 2 and the peripheral portion 4 of the cavity 1 is smoothly changed, as shown by an arrow a, along the wall surface of the peripheral portion 4 toward the center portion 3. Further, it may be necessary that the fuel spray A flowing toward the center of the cavity 1, which is shown by an arrow b, is mixed with the air sufficiently.